(a) Field of the Invention
The present invention relates to polymer compounds and a preparation method thereof. More specifically, the present invention relates to polymer compounds with well-connected, narrow size distribution free-volume element and a method for preparing the polymer compounds by thermal rearrangement of aromatic polyimides containing ortho-positioned functional groups in the solid state.
(b) Description of the Related Art
Small-molecule and ion diffusion through cavities (i.e., free-volume elements) in soft organic materials is an inherently subnano- or nanoscopic phenomenon. It has important implications for membrane separation processes in chemicals production as well as energy conversion and storage applications [e.g., pharmaceutical separations (B. Jeong et al., Nature 1997, 388, 860), organic batteries (P. Lightfoot et al., Science 1993, 262, 883), fuel cells (M. A. Hickner et al., Chem. Rev. 2004, 104, 4587), and gas separation (H. Lin et al., Science 2006, 311, 639)]. Transport of small gas molecules through polymers occurs by diffusion through transient free-volume elements or cavities formed by random, thermally stimulated motion of the flexible organic chains.
Unlike pore sizes and shapes in rigid microporous inorganic materials such as zeolites (Z. Lai et al., Science 2003, 300, 456) and carbon molecular sieve materials (H. B. Park et al., Adv. Mater. 2005, 17, 477), cavity sizes and shapes are not uniform in amorphous polymers. The cavity radius (r) of the most selective polymers such as polyimides, polysulfones, and polycarbonates, as measured by positron annihilation lifetime spectroscopy (PALS), is 0.3 nm or less with a broad distribution of cavity sizes, and gas permeability is rather low (Y. Yampolskii, I. Pinnau, B. D. Freeman, Materials Science of Membranes for Gas and Vapor Separation (Wiley, London, 2006).
Conversely, the most permeable polymer, poly(1-trimethylsilyl-1-propyne) (PTMSP), exhibits an approximately bimodal cavity size distribution centered at around r=0.3 nm and r=0.6 to 0.7 nm (K. Nagai et al., Polym. Sci. 2001, 26, 721). The high concentration of large cavities and the high connectivity among cavities results in very high permeability for a polymer, but its ability to separate small molecules (kinetic diameter <0.45 nm) is too low to be useful, and the large cavities collapse over time due to physical aging (K. Nagai et al., Polym. Sci. 2001, 26, 721). Thus, among known polymers, free-volume element size and distribution play a key role in determining permeability and separation characteristics. However, the broad size range of free-volume elements in such materials precludes the preparation of polymers having both high permeability and high selectivity.
The inventors of the present invention demonstrate that polymers with an intermediate cavity size, a narrow cavity size distribution, and a shape reminiscent of bottlenecks connecting adjacent chambers, such as those found elegantly in nature in the form of ion channels (D. A. Doyle et al., Science 1998, 280) and aquaporins (D. Kozono et al., Invest. 2002, 109, 1395), yield both high permeability and high selectivity. Central to approach for preparing these intermediate-sized cavities is controlled free-volume element formation through spatial rearrangement of the flat, rigid-rod structure with high-torsional energy barriers to rotation between two rings (V. J. Vasudevan, J. E. McGrath, Macromolecules 1996, 29, 637). The stiff, rigid ring units in such flat topologies pack efficiently, leaving very small penetrant accessible free-volume elements. This tight packing is also promoted by intersegmental interactions such as charge-transfer complexes between heteroatoms containing lone electron pairs (e.g., O, S and N) (W. J. Welsh, D. Bhaumik, J. E. Mark, Macromolecules 1981, 14, 947). The genesis of these materials was the demand for highly thermally and chemically stable polymers. However, their application as gas separation membranes was frustrated by their lack of solubility in common solvents, which effectively prevents them from being prepared as thin membranes by solvent casting, which is the most widely practiced method for membrane preparation.
Consequently, the present inventors suggested that completely aromatic, insoluble, infusible polymers can be prepared from highly soluble precursors by irreversible molecular rearrangement at about 350° C. to 450° C. for aromatic polyimides containing ortho-positioned functional groups (e.g., —OH, —SH and —NH2) [H. B. Park, C. H. Jung, Y. M. Lee et al., Polymers with cavities tuned for fast selective transport of small molecules and ions, Science 2007, 318, 254. 38]. In addition, the present inventors ascertained that aromatic polymers interconnected with heterocyclic rings (e.g., benzoxazole, benzothiazole and benzopyrrolone) showed higher gas permeation performance due to their well-controlled free-volume element formation by thermal rearrangement in the solid state.